1. Field of the Invention
The present invention relates to an optical unit including a high-precision objective unit having a lens, a lens frame, and a lens barrel and a method of assembling the optical unit.
Further, the present invention relates to an optical unit having a lens-holding apparatus suitable for a high-precision objective lens, etc.
2. Description of the Related Art
Conventionally, as shown in FIG. 7, a microscope objective unit includes a plurality of lenses 103a through 103f, a plurality of lens frames 102a through 102d for holding the lenses 103a through 103f, and a lens barrel 101.
The lens frames 102a through 102d have substantially the same outside diameter. The lens frames 102a through 102d hold lenses 103a through 103f. The lens frames 102a through 102d have center axes for the outside diameters. The center axes of the lens frames 102a through 102d substantially correspond to optical axes of the lenses 103a through 103f. 
The lens frames 102a through 102d have center axes for the outside diameters. The center axes of the lens frames 102a through 102d substantially correspond to optical axes of the lenses 103a through 103f. 
The lens barrel 101 has the center axis. The lens barrel 101 is cylindrical and has a hole 101a along the center axis. The hole 101a has an inside diameter substantially the same as the outside diameter of the lens frames 102a through 102d. 
The lens frames 102a through 102d holding the lenses 103a through 103f are inserted into the hole 101a. The center axes of the lens frames substantially correspond to the center axis of the lens barrel 101. Consequently, optical axes of the lenses 103a through 103f substantially correspond to each other in the hole 101a. 
The objective unit is assembled by inserting the lens frames holding the lenses into the lens barrel. This assembling method can provide centering more accurately than a method of assembling the objective unit by directly dropping the sets of lenses 103a through 103f into the lens barrel 101.
There is provided a plurality of types of lens frames as means for holding lenses. For example, the holding means is available as a lens frame as shown in FIG. 8. FIG. 8 shows an optical unit including the above-mentioned lens frame. The optical unit has a set of lenses including lenses 103g and 103h. In the set of lenses, optical axes of the lenses 103g and 103h are coaxially arranged. In FIG. 8, the reference numeral 105 represents an optical axis of the set of lenses.
A lens frame 102e has a predetermined outside diameter. The lens frame 102e has a center axis for the outside diameter. The lens frame 102e has a lens frame edge 104 orthogonal to the center axis. The lens frame edge 104 is formed with high precision along a direction orthogonal to the center axis. The lens frame 102e has a stop which extends toward the inside of the frame itself along a direction orthogonal to the center axis. The stop has a retaining surface for seating a lens.
When the lens frame 102e is used to hold lenses, a set of lenses (cemented lens) including the lenses 103g and 103h is dropped onto the retaining surface in the lens frame 102e. The set of lenses is held on the retaining surface of the lens frame 102e. The set of lenses is fixed to the retaining surface with adhesive.
When adhesively bonding the above-mentioned lens set to the lens frame 102e, the lens 103h is first placed on the retaining surface. The lens frame 102e is then moved along the lens frame edge 104. More specifically, the lens frame 102e rotates around its center axis on the lens frame edge 104. The lens 103h is centered with respect to the lens frame 102e by the rotating. The lens frame 103h is then adhesively bonded to the lens frame 102e. Likewise, the lens 103g is arranged on the lens 103h and is centered with respect to the lens frame 102e. The lens 103g is then adhesively bonded to the lens 103h. The lens frame 102e having the retaining surface is often used as a lens holding means for ordinary objective units.
FIG. 9 shows a lens frame 102f as another example of the above-mentioned holding means. The lens frame 102f in FIG. 9 is configured to be capable of plastic deformation. The lens frame 102f is configured to include a lens 103i. The lens frame 102f is caulked at its end to fix the lens 103i. Accordingly, the lens frame 102f can fix the lens 103i without using adhesive. The means for caulking the lens frame 102f has been long used.
FIG. 10 shows a holding means capable of maintaining a clearance along optical axes of two lenses. FIG. 10 also shows an optical unit including the above-mentioned holding means. The optical unit has a set of lenses A comprising a convex lens 31 and a concave lens 32 bonded to each other and a set of lenses B comprising a convex lens 33 and a concave lens 34 bonded to each other.
The holding means in FIG. 10 has a lens frame 144 for holding the set of lenses A and a lens frame 146 for holding the set of lenses B. The holding means further has a clearance ring 145 arranged between the lens frames 144 and 146. The clearance ring separates the lens frames 144 and 146 with a predetermined clearance along center axes of the lens frames 144 and 146. Consequently, the sets of lenses A and B can separate predetermined clearance along the optical axes thereof.
The set of lenses in FIG. 8 including the lenses 103g and 103h is dropped into the lens frame 102e to be seated in the lens frame 102e. Accordingly, the lens frame 102e can be bonded while the set of lenses is centered. The lens frame 102e needs to have a stop because the set of lenses is dropped thereinto. The space for providing the stop is needed around the lens frame 102e. 
As shown in FIG. 11, however, there is no space around the lens frame 102e for an optical system in which lenses are arranged close to each other. It is difficult for the lens frame 102e to maintain or bond the optical system in FIG. 11.
The lens frame 102f in FIG. 9 holds the lens 103i by caulking as mentioned above. Generally, the lens frame 103f it caulked at a very small portion thereof. Thus this caulking is difficult. When the lens 103i is fixed by caulking, the accuracy of centering depends on the worker""s experience and skill. When optical unit are manufactured through the use of caulking, there is a problem of widely varying the quality of the optical unit.
As mentioned above, the holding means in FIG. 10 has a clearance ring 145 in addition to the lens frames 144 and 146. Since the holding means has a plurality of members, the structure is complicated. Accordingly, it is difficult to improve the accuracy of centering for the set of lenses A and B. Since the holding means has the complicated structure, a clearance between the set of lenses may not be provided highly precisely.
As shown in FIG. 12, there is devised a holding means having a simplified structure. The simplified structure is provided by integrating the lens frame 144 and the clearance ring 145 in FIG. 10. In FIG. 12, the reference numeral 148 represents a lens frame formed by integrating the lens frame 144 and the clearance ring 145. The use of the lens frame 148 simplifies the optical unit configuration. However, the lens frame 148 has a larger dimension in the direction along its center axis than that of the lens frame 144. Accordingly, the set of lenses A is arranged at an inner part in the direction along the center axis of the lens frame 148. The lens frame 148 makes it difficult to center the set of lenses A.
Especially, a microscope objective unit using wavelengths in an ultraviolet range uses more lenses than a microscope objective unit using wavelengths in a visible range for the following reason.
An ordinary glass does not transmit the light having a wavelength of 300 nm or less. Accordingly, lenses for the ultraviolet range have a limitation on the use of a glass material such as fluorite or quartz which can transmit a wavelength of 300 nm or less.
Therefore, an objective unit for the ultraviolet range is disadvantageous with respect to the correction of chromatic aberration.
In order to conduct the correction of chromatic aberration, it is necessary to arrange objective unit lenses adjacently to each other. One method of arranging the objective unit lenses adjacently to each other is to use an adhesive to bond these lenses to each other. It should be noted that the ultraviolet light degrades the adhesive. Since the transmittance of the bonded lenses decreases, it is undesirable to use an adhesive.
For the correction of chromatic aberration, it is therefore preferable that the lenses are separated from each other with a predetermined interval. Widening an interval between lenses decreases the effect of the correction of chromatic aberration. It is desirable to adjacently arrange convex and concave lenses having different medium.
For the reason as mentioned above, the lenses are very closely arranged in the microscope objective lens unit using wavelengths in the ultraviolet range as shown in FIG. 11. In surfaces of the lenses facing to each other in FIG. 11, curvature radius Rp of the convex lens approximately equals curvature radius Rn of the concave lens. In order to provide substantially the same curvature radius, Rp/Rn must be set to satisfy the following condition.
(Condition) 0.58 less than Rp/Rn less than 1.65
When the curvature radius of each facing surface satisfies this condition, the objective unit comprising the above-mentioned lenses can appropriately correct aberrations including the chromatic aberration. When the above-mentioned Rp/Rn is exceeded from 0.58 less than Rp/Rn less than 1.65, the correction of chromatic aberration, in particular, becomes difficult.
When there are many lenses close to each other, holding the lenses is difficult for the conventional holding means as shown in FIG. 8. The surface shape of the lens is very accurately in the microscope objective unit using wavelengths in a deep ultraviolet range. Accordingly, holding the lenses is difficult for the holding means as shown in FIG. 9. The microscope objective unit requires high precision for a clearance between lenses. It is also difficult to use the holding means as shown in FIG. 10. These points represent a first problem.
In consideration of the first problem, it would be desirable to provide an optical unit which has a lens frame capable of holding the set of closely arranged lenses and is capable of improve the accuracy of centering and appropriately maintaining the arrangement of the set of lenses with respect to the lens frame, that is the positional relationship between the set of lenses and the lens frame.
Further, it would be desirable to provide assembling an optical unit which has a lens frame capable of holding a set of closely arranged lenses and is capable of improve the accuracy of centering and appropriately maintaining the arrangement of the set of lenses with respect to the lens frame, that is the positional relationship between the set of lenses and the lens frame.
Generally, when lenses are directly fixed in a lens barrel, the optical unit makes it difficult to arrangement the lenses precisely to the barrel. For this reason, the optical unit uses a lens-holding apparatus for holding lenses. The lens-holding apparatus holds the lenses via a lens frame as shown in FIG. 18. For example, the lens-holding apparatus is used to highly accurately hold set of lenses such as an objective unit. FIG. 18 shows an ordinary lens-holding apparatus.
The optical unit such as an objective unit of a microscope has a plurality of lenses. For example, the optical unit in FIG. 18 has three lenses 1101, 1102, and 1103. In this optical unit, errors on decetering or decentration for the lenses 1101, 1102, and 1103 greatly affect the optical characteristics. Accordingly, the lenses 1101, 1102, and 1103 are assembled so as to improve accuracy of optical centers of lens frames 1111, 1112, and 1113 for holding the respective lenses. As a result, the optical center of each lens can maintain a predetermined accuracy with reference to a lens barrel 1120. This can provide Optical unit (objective unit) comprising the lens-holding apparatus with intended optical characteristics.
The above-mentioned conventional lens-holding apparatus has the following problem. If there is a small clearance C between lenses along the optical axis in FIG. 18, the lenses interfere with each other during assembly. Further, for example, FIG. 19 shows a structure in which a lens clearance is small not only in a direction along the optical axis, but also in a direction crossing the optical axis. Such structure increases the risk of interference between lenses. FIG. 19 shows that the lenses 1102 and 1103 having substantially the same radius of curvature are arranged apart from each other with clearance C in the thrust direction along the optical axis. Clearance B is a minimum gap between the lenses 1102 and 1103 along the perpendicular direction (radial direction) of the optical axis.
Generally, during a manufacturing process of the optical unit, a lens frame 1112 holding a lens 1102 is slightly but frequently moved in the radial direction (indicated by an arrow in FIG. 20) against a lens frame 1113 holding a lens 1103. In FIG. 20, lenses 1102 and 1103 are coaxially arranged. When the lens frame 1112 is located with a predetermined decentration. (indicated by X in FIG. 21) with respect to the lens frame 1113, the lenses interfere with each other at an interference portion (indicated by a broken line in FIG. 21). This interference of lenses 1102 and 1103 flaws the lens surface of lenses 1102 and 1103 unlike contact of metallic lens frames with each other. The flawed lens may cause a defect in the performance and the appearance. Especially, a convex lens such as the lens 1103 is often formed of relatively soft glass material such as CaF2 (fluorite) according to the optical design. Such the convex lens is flawed by a small amount of the interference easily.
The flawed lens is defective. Even though the lens does not become defective, special care must be taken to handle such lens during the manufacturing process. Consequently, the productivity of such optical unit is degraded. These points constitute a second problem.
In consideration of the above-mentioned second problem in the prior art, it would be desirable to provide an optical unit including a lens-holding apparatus which prevents the lenses from being damaged by interference of the lenses and is easily handled in the manufacturing process.
An optical unit according to a first aspect of the invention comprises at least one lens having an outer surface; at least one lens frame to hold the lens. The lens frame includes a cylindrical portion having an edge and an inner surface to contact the outer surface of the lens; and a U-shaped portion to hold the lens. The U-shaped portion has a U-shaped cross-section, and is formed integrally with the cylindrical portion at the edge thereof; so that the lens held by the U-shaped portion is adhesively bonded to the cylindrical portion and then the U-shaped portion is removed from the cylindrical portion.
A method of assembling an optical unit according to another aspect of the invention is assembling the optical unit. The optical unit comprises a lens, a cylindrical lens frame, and a cylindrical lens barrel. The lens includes an outside diameter, an outer periphery having a center axis, and an optical axis coaxially arranged to the center axis. The cylindrical lens frame holds the lens. The frame includes an outside diameter, an outer periphery having a center axis, a cylindrical portion and a center axis. The cylindrical portion has a center axis and substantially the same inside diameter as the outside diameter of the lens. The center axis 6f the frame coaxially arranged to the center axis of the outer periphery of the cylindrical portion. The cylindrical lens barrel includes a barrel hole having substantially the same an inside diameter as the outside diameter of the lens frame, the barrel hole having a lens-holding surface with which the frame is to be contact. The cylindrical portion has one opening end and the other opening end along the center axis. The cylindrical portion has a U-shaped portion. The U-shaped portion has a U-shaped cross-section and a lens-retaining surface for positioning the lens along the optical axis. The optical unit assembling method comprises removing the U-shaped portion from the cylindrical portion after adhesively bonding the lens to the cylindrical portion, and fixing the lens frame to the lens-holding surface after removing the U-shaped portion from the cylindrical portion.
A method of assembling an optical unit according to yet another aspect of the invention mounts a lens frame to hold a lens in a lens barrel. The lens has optical axis. The lens frame has a lens-holding surface for holding an outer surface of a lens. The assembling method comprises forming a U-shaped portion to the lens frame. The U-shaped portion has a U-shaped cross-section and a lens-retaining surface for positioning the lens along the optical axis.
In addition, the assembling method comprises bonding the lens on the lens frame by adhesive, removing the U-shaped portion from the lens frame, and mounting the lens frame in the lens barrel.
An optical unit according to still another aspect of the invention comprises a plurality of lenses, each of the lenses having a lens surface and an optical axis, and a lens-holding apparatus. The lens-holding apparatus includes a plurality of lens frames holding at least one lens respectively, and a lens barrel. The lens frames have portions overlapping with each other in a radial direction orthogonal to the optical axis when the lens frames are arranged adjacent each other along the optical axis. The lens-holding apparatus is provided with first clearance in the adjacent two lens frames between the overlap portion of one lens frame and the overlap portion of the other lens frame in a radial direction orthogonal to the optical axis. The lens-holding apparatus is provided with second clearance in the radial direction between the lens surfaces of lenses held by the adjacent two lens frames. The first clearance is smaller than the second clearance. The lens frames is arranged in the lens barrel.
An optical unit according to still another aspect of the invention comprises the lenses, and a lens-holding apparatus. The lenses have an outer surface, a lens surface, and an optical axis. The lens-holding apparatus includes a plurality of lens frames and a lens barrel. The lens frames holds at least one lens respectively. The lens frame includes a cylindrical portion and a U-shaped portion. The cylindrical portion has an edge and an inner surface to contact the outer surface of the lens. The U-shaped portion to holds the lens has a U-shaped cross-section, the U-shaped portion being formed integrally with the cylindrical portion at the edge thereof; so that the lens held by the U-shaped portion is adhesively bonded to the cylindrical portion and then the U-shaped portion is removed from the cylindrical portion. In addition, the lens frames have portions overlapping with each other in a direction along the optical axis when the lens frames are arranged adjacent each other along the optical axis. The lens-holding apparatus is provided with first clearance in the adjacent two lens frames between the overlap portion of one lens frame and the overlap portion of the other lens frame in a radial direction orthogonal to the optical axis. The lens-holding apparatus is provided with second clearance in the radial direction between the lens surfaces of lenses held by the adjacent two lens frames. The first clearance is smaller than the second clearance. The lens frames is arranged in the lens barrel.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.